Generate multiple macro calls according to the number of arguments

Question

I'm trying to call this function several times in a repeditive fashion.

template<class T>
void METADATA_METHODS_IMPL(std::string& metadata, const T &value, const std::string &key)
{
    metadata += boost::format("%1%:%2%") % key % value();
}

I have written the following macro:

#define METADATA_METHODS_IMPL_1(md, v1)\
   METADATA_METHODS_IMPL(md, v1, #v1);

#define METADATA_METHODS_IMPL_2(md, v1, v2)\
   METADATA_METHODS_IMPL_1(md, v1)\
   METADATA_METHODS_IMPL_1(md, v2)

#define METADATA_METHODS_IMPL_3(md, v1, v2, v3)\
   METADATA_METHODS_IMPL_2(md, v1, v2)\
   METADATA_METHODS_IMPL_1(md, v3)

and so on ...

I need to call METADATA_METHODS_IMPL_N where N is a number of arguments in __VA_ARGS__

#define METADATA_METHODS(...)\
std::string METADATA_METHODS_IMPL_FUNC()\
{\
    std::string metadata;\
    BOOST_PP_OVERLOAD(METADATA_METHODS_IMPL_,__VA_ARGS__)(metadata, __VA_ARGS__)\
    return metadata;\
}

The code above (with BOOST_PP_OVERLOAD) gives me unsuitable result:

class X
{
    std::string F1();
    std::string F2();
    std::string F3();

    METADATA_METHODS(F1, F2, F3);
};

This results in

std::string METADATA_METHODS_IMPL_FUNC() 
{
    std::string metadata; 
    METADATA_METHODS_IMPL(metadata, F1, F2, F3, "F1", "F2", "F3"); 
    METADATA_METHODS_IMPL(metadata, , ""); 
    METADATA_METHODS_IMPL(metadata, , ""); 
    return metadata; 
};

And I want something like this:

std::string METADATA_METHODS_IMPL_FUNC() 
{
    std::string metadata; 
    METADATA_METHODS_IMPL(metadata, F1, "F1"); 
    METADATA_METHODS_IMPL(metadata, F2, "F2"); 
    METADATA_METHODS_IMPL(metadata, F3, "F3"); 
    return metadata; 
};

• Does anyone know how to achieve the desired result?
• Can I use Boost.preprocessor library to automagically generate METADATA_METHODS_IMPL_K for some K in [1 .. 10] using METADATA_METHODS_IMPL_1

Answer 1

Using these macros we can cause the preprocessor to keep rescanning and allow for recursive macros.

#define EVAL(...)  EVAL3(EVAL3(EVAL3(__VA_ARGS__)))
#define EVAL3(...) EVAL2(EVAL2(EVAL2(__VA_ARGS__)))
#define EVAL2(...) EVAL1(EVAL1(EVAL1(__VA_ARGS__)))
#define EVAL1(...) EVAL0(EVAL0(EVAL0(__VA_ARGS__)))
#define EVAL0(...) __VA_ARGS__

We can define some helper functions to build up concepts.

#define CAT(a, ...) PRIMITIVE_CAT(a,__VA_ARGS__)
#define PRIMITIVE_CAT(a, ...) a ## __VA_ARGS__

#define EMPTY()
#define EAT(...)
#define IDENT(...) __VA_ARGS__
#define DEFER(id) id EMPTY()
#define OBSTRUCT(...) __VA_ARGS__ DEFER(EMPTY)()   

#define I_TRUE(t,f) t
#define I_FALSE(t,f) f
#define I_IS_DONE(a,b,...) b
#define TRUE_DONE() ~, I_TRUE
#define IS_DONE(b) OBSTRUCT(I_IS_DONE)(CAT(TRUE_,b)(),I_FALSE) 

And build this Macro Mapping function that carries the first parameter to each.

#define MM() MM_CALL
#define MM_NEXT(Macro,md,a,...) \
  IS_DONE(a)(                    \
     EAT                          \
  ,                                \
     OBSTRUCT(MM)()                 \
  )                                  \
  (Macro,md,a,__VA_ARGS__)
#define MM_CALL(Macro,md,a,...)   \
  Macro(md,a)                      \
  MM_NEXT(Macro,md,__VA_ARGS__)
#define MacroMap(Macro,md,...) EVAL(MM_CALL(Macro,md,__VA_ARGS__,DONE))

After defining the implementation related functions

#define METADATA_METHODS_IMPL_MACRO(md,a) \
  METADATA_METHODS_IMPL(md, a, #a);

#define METADATA_METHODS(md,...) \
  MacroMap(METADATA_METHODS_IMPL_MACRO,md,__VA_ARGS__) \
  return md

This:

METADATA_METHODS(metadata, F1, F2, F3);

Results in this (after adding some formatting):

METADATA_METHODS_IMPL(metadata, F1, "F1");
METADATA_METHODS_IMPL(metadata, F2, "F2");
METADATA_METHODS_IMPL(metadata, F3, "F3");
return metadata;